1. Field of the Invention
The present invention relates to a method of charging a battery and a battery charging control system, and particularly to a method of charging a battery such as a lithium ion battery at a low temperature for fast charging and a battery charging control system for fast charging.
2. Description of the Related Art
Vehicles such as hybrid electric vehicles (HEV) and electric vehicles (EV) travel by driving a motor using electric power from a battery. When a remaining charge in the battery decreases by discharging during, for example, traveling and a long period parking, the battery should be charged. Because quick charging may result in deterioration and shortened life time of the battery, it is recommended to charge the battery with a charging current corresponding to one-tenth of a rated capacity (0.1 C) for about ten hours from the viewpoint of protection of the battery. Since the battery at a low temperature has a high impedance, charging time can be shortened by heating the battery with a heater or the like before start of charging. Such a charging method using external heating is undesirable in consideration of layout in the vehicle because an external heating device should be mounted on the vehicle and an additional power source for a heater becomes necessary.
JP 2002-125326 A discloses a technology of controlling charging a battery in which pulse charging and discharging is performed to heat the battery and then the charging mode is shifted to a normal charging mode. According to the technology, when a battery temperature is a considerably low, (for example, at or below 0° C.), a first pulse charging and discharging with the same charge quantities is performed to heat the battery. When the battery temperature becomes slightly higher (for example, at 0° C. to 10° C.), both heating and charging are done substantially simultaneously by a second pulse charging and discharging, wherein the discharging quantity is smaller than the charging quantity. After the battery temperature reaches a predetermined temperature, the charging mode is shifted to the normal charging mode.
In the method of charging the battery disclosed by JP 2002-125326 A in which the pulse charging and discharging is repeated before the normal charging mode to increase the temperature of the battery at the low temperature, it is necessary to previously determine a pulse width and charging and discharging current quantity for the pulse charging and discharging to prevent the charging voltage from reaching an upper voltage of the battery through a pretest. The upper voltage of the battery, for example, in a case of a lithium base battery, corresponds to a highest voltage allowing no decomposition of a battery electrolyte. Actually, electrodes of the battery may be damaged before the battery voltage reaches the upper limit by causes such as deposition of lithium (Li) on a negative electrode of the lithium base battery. Therefore, the pulse width for the pulse charging and discharging should be determined through a pretest to perform the charging and discharging at a voltage slightly lower than the upper limit voltage. This operation results in poor usability for charging the battery.
The present invention provides a method of charging a battery and a battery charging control system capable of charging after appropriately heating the battery by a charging and discharging control with a threshold in pulse duration relating to avoidance of electrode damage.